1. Field of the Invention
The present invention relates to a system and a method for reducing the crest factor of a signal, and more particularly, to a system and a method for reducing the crest factor of a signal which can reduce the crest factor without having information about an individual channel and be applied to baseband, IF and RF signals.
2. Related Prior Art
A crest factor is an important characteristic of an RF signal in the transmission of the RF signal. The crest factor is a ratio of a peak value to an effective value of a signal waveform. That is, the crest factor is obtained by dividing a peak value of a signal by an effective rms value of the signal, and represents a degree of sharpness of a wave front. The crest factor is represented as follows.
                              CF          ⁡                      (                          Crest              ⁢                                                          ⁢              Factor                        )                          =                  PeakValue          RMSValue                                    [                  Equation          ⁢                                          ⁢          1                ]            
For example, a sine wave has a crest factor of √{square root over (2)} and a random wave can theoretically have a crest factor of an infinite value.
The crest factor is determined by a peak-to-average voltage ratio in the transmission of a multi-carrier of a digital signal. That is, the crest factor is proportional to the square root of a peak-to-average power ratio of a transmission signal.
Conventional techniques for reducing the crest factor of a signal include a clipping method and a baseband control method.
In the clipping method, a baseband signal has its amplitude clipped at a constant level. The clipping method removes the peak of a signal to reduce the crest factor of the signal. Though clipping is a simple technique because it is nonlinear processing, it should be carefully used. For example, the filter for filtering a clipped signal must be carefully selected.
The baseband control method controls the frequency band of an original signal before the modulation of the frequency band for the transmission of the signal, in order to reduce the crest factor of the signal. This method requires information about a transmission channel.
FIG. 1 is a block diagram of a conventional system and method for reducing the peak-to-average power ratio of a multi-carrier signal, which is disclosed in U.S. Pat. No. 6,314,146. In FIG. 1, reference numeral 840 denotes a receiver and a block 855 estimates a signal distorted by clipping in order to restore an original signal. This technique clips the original signal at transmission to reduce the peak-to-average power ratio of the signal. When the signal is clipped to a distorted form and transmitted to the receiver, the receiver makes an estimate of the clipped portions of the signal in order to restore the original signal. At this time, the receiver also receives information about signal clipping (clipping size, clipping peak and so on). This technique reduces the peak-to-average power ratio without having the loss of bandwidth of the original signal.